This invention relates generally to apparatus and methods for removing contaminating mists and particulates from gases in which such agents are dispersed, and more specifically relates to filtration devices and methods for removing oil mists and other organic vapor mists, as well as other solid and liquid particles from airstreams in which they are dispersed and/or with which they form an aerosol.
Oil mist and organic vapor mists are very difficult to capture once airborne. By the term xe2x80x9cmistxe2x80x9d is meant a dispersion in gas (typically air) of liquid droplets sufficiently small to form a true suspension under ambient conditions, as well as dispersals of droplets above such size. Many industrial operations generate such oil mists, airborne hydrocarbon mists and volatile organic compound (VOC mists), resulting in harmful and costly pollution. Such pollution can affect indoor/outdoor air quality and create odor problems, generate HVAC/heat exchange fouling, cause gross contamination of roof-tops, and generate hazardous stormwater run-off and ground water contamination. Facilities experiencing these problems include metal working operations (machining and tooling), surface coating operations (solvent-based industrial painting), auto manufacturing operations, and oil refining and chemical production operations. Similarly compressors or the like associated with HVAC systems of buildings or even sea vessels often introduce oily mists into the HVAC system, and ultimately into the ambient environment.
In addition to liquid droplets of the type discussed above, solid particles dispersed in gases such as air, represent further common contaminants which contribute to pollution, and are often sought to be removed. Such solid particles are often present along with separate liquid droplets or particles. In other instances the liquid and solid particles can be present as combined entities, e.g., where the solid particles are enveloped or coated by the liquid.
A more general concern arises where one is concerned with filtration of complex waste streams where oil mist and suspended solid particles are present simultaneously, either as or with true aerosols and in the presence of suspended particulate matter which has been rendered static dissipative and/or neutral, which reduces the strength of the van der Waal""s affinities. Conventional adsorbents and filter media rely heavily on such van der Waal""s forces and other so called weak molecular interactions in order to perform as expected.
Conventional treatment methods of air streams contaminated with low concentrations of volatile organic compound (VOCs) mists, hydrocarbon and/or oil vapor mists, are expensive, complicated and difficult to operate. Yet many industries generate such air streams which must therefore be treated to meet the requirements of the Montreal Protocol and Clean Air Act Amendments of 1990. Conventional treatment technologies for removal of low mist concentrations from air streams include intense physical or chemical process such as multi-layer adsorption, catalytic conversion and photolytic degradation. Attempts have been made using bioscrubbers and/or an air phase bioreactors to remove hydrocarbons, but they are only able to achieve up to 75% removal because of flowrate/contact time issues, and performance varies significantly depending upon superficial gas velocity, gas inlet concentration, mass transfer coefficients and biokinetic constants.
In 1995, to comply with new EPA standards for organic air emission for hazardous waste treatment, storage and disposal facilities (TSDF""s) and hazardous waste generators, Lawrence Livermore National Laboratory (LLNL) designed an organic removal and destruction treatment train to modify its existing wastewater treatment tank farm and achieve 95% TOC reduction. It consists of an air stripper, high-efficiency particulate air filter (HEPA), catalytic oxidizer, scrubber and mist eliminator.
The original high performance filters referred to as HEPA (High Efficiency Particulate Arrestors) were developed during the Manhattan project to prevent discharge of radioactive particles. Since then they have become the preeminent technology when high efficiency filtration is required in industry, medicine, military applications and more recently for household filtration devices. HEPA filters are composed of submicron glass fibers. A HEPA filter by definition will have a 99.97% particle removal efficiency for all particles of 0.3 micron diameter. HEPA filters exhibit higher efficiency for both smaller or larger particles. HEPA filters have high pressure drop performance characteristics and usually require a prefilter for optimum performance.
Similarly, work has been done with granulated activated carbon (GAC), but it has numerous well-known drawbacks such as clogging, re-release and absorption capacity. Utilizing zeolites in multi-layer design with GAC, while generally effective, is expensive and multi-layers cause large pressure drops (xcex94 P) across filters based upon these combinations.
For purposes of this specification the term xe2x80x9cdispersedxe2x80x9d in reference to any of the several above types of particle/gas systems means that the mist, or solid particles etc. are dispersed in a gaseous phase such as air. The term xe2x80x9caerosolxe2x80x9d can be defined as a system of solid or liquid or liquid-enveloped solid particles suspended in a gaseous medium, the particles having a negligible falling velocity.
Aerosols are further characterized herein by having liquid or solid (or combined liquid/solid) particles of less than 100 microns diameter. In a typical distribution, at least 40% by weight of the particles are less than 0.2 micron. At this size, assuming low surface charge which inclusion of organic and ionic compounds will promote, these droplets and/or particulates will stay buoyant in air indefinitely. The strength of the surface tension in a droplet is more than 100,000 times the mass of the droplet. Unless the droplet is opened, the pollutants entrapped inside remain inaccessible. It is been shown that when liquid aerosols are recirculated through a filter the droplet size distribution becomes much narrower and tends towards the smaller micron ranges. In other words if a liquid aerosol is not adsorbed onto a filter surface, smaller more robust droplets are formed with much higher surface tension to mass ratios.
Aerosols are formed spontaneously under ambient conditions in multiple ways. Particulate matter provides nucleation centers around which liquids and organic compounds self-assemble. Human activities like talking, breathing, and sneezing create liquid aerosols. This is the vector for transmission for all cases of tuberculosis. Movement provides particulate nucleation centers and household systems which move air create oily aerosol droplets. Other pollutants spontaneously coalesce and adhere to these initial aerosol droplets.
Air pollution in fact exists in many forms and each of these have different surface characteristics and affinities. A somewhat more complete picture of the complex interactions which occur in air is as follows:
Primary mechanisms of air pollution formation include:
1. Formation of volatile organic compounds through evaporation and incomplete combustion.
2. Biological generation of volatile organic compounds.
3. Formation of liquid aerosol droplets through shear and turbulence related processes, from cooking and household solvent based cleaners.
4. Anthropomorphic generation of particulate aerosol.
5. Biological formation of particulate aerosols through decomposition and spore formation.
6. Particulate aerosol formation from geological and weather based phenomena.
Secondary mechanisms of air pollution formation include:
1. Condensationxe2x80x94Liquid aerosol droplet formation due to condensation of gaseous phase components.
2. Adsorptionxe2x80x94Adsorption of gaseous phase and liquid aerosol components onto particulate surfaces resulting in particulate aerosols with modified surfaces from adsorption of organic compounds and hydrous inorganic phases.
3. Nucleationxe2x80x94Liquid aerosol formation through precipitation onto micro particulate nucleation sites.
The above sources and processes will result minimally in the following general categories of airborne pollutants based on surface characteristics and affinities. This is important because the components of a filtration device must have complementary affinities:
1. Particulate aerosol (charged or polar surface)
2. Particulate aerosol (neutral or conductive surface)
3. Liquid aerosol (organic)
4. Liquid aerosol (hydrous)
5. Liquid aerosol (mixed)
Ambient air pollution is a hydra-like beast. Mechanisms such as adsorption, nucleation and condensation are capable of producing a variety of airborne species with different surface affinity and bulk characteristics. Some of the forms of ambient air pollution documented to date are as follows:
1. Dry and hydrous carbonaceous material resulting from anthropomorphic and natural processes.
2. Inorganic micro particulates such as aluminium oxide, silicates and other compounds resulted from geological processes.
3. Spores, pollen seeds of biological origin.
4. Endotoxins and other detritus resulting from biological decomposition processes.
5. Liquid non polar aerosols (all liquid).
6. Liquid polar hydrous aerosols (all liquid).
7. Liquid aerosol with particulate nucleation sites.
8. Particulate aerosol with adsorbed non polar liquid organic constituents on surface.
9. Particulate aerosol with adsorbed hydrous liquids (polar).
10. Particulate aerosol with mixed emulsion of hydrous and non polar constituents.
It is much harder to remove pollutants which are entrapped in aerosol droplets than it is to remove single source free flowing compounds. The filter material must have complementary affinity for the surface of the suspended aerosol in order to entrap particulate aerosol material and in the case of liquid aerosols in order to reduce the surface energy at the interfacial membrane in order to expose the pollutants entrapped within the droplets.
In the case of the particulate aerosols the most difficult to remove tend to be less than 3 microns in size and to have a neutral or insulating surface. In this case filtration efficiency can be increased by modifying the filter substrate surface to have affinity for the non polar particle surface. Without filter affinity if one relies wholly on physical filtration mechanisms untenable differential pressures quickly develop across the system.
Most air filters are porous fibrous materials with fibers aligned perpendicular to air flow. Fiber diameters are typically less than 0.50 mm and the resultant interfibrous spaces are usually much larger than the airborne pollutants to be filtered out. More than 90% of the filter volume is void space. Direct interception does not occur frequently. At particle sizes smaller than 0.6 micron Brownian diffusion is the predominant filtration mechanism with electrostatic interactions having an effect in some cases. At over 0.6 micron inertial impaction and gravity settling predominate. In 1942, Irving Langmuir calculated the particle size where the contribution of the above effects would be at a minimum. The result of this theoretical calculation was that minimum filter efficiency would occur at 0.3 micron. Consequently filters are still evaluated based on performance at the point of minimum efficiency as calculated by Langmuir. Interestingly many of the air filter performance standards and analytical techniques and equipment employed today were developed during World War II as a part of personal protective mask research and chemical fog and chemical warfare research in addition to development of HEPA filters (High Efficiency Particulate Arrestors) for capture of radioactive materials.
Due to this early work and subsequent research air filter performance has become better understood. Standardized tests have been developed to gauge filter performance. One of the more common standards which is utilized is the MERV (Minimum Efficiency Reporting Value) rating system based on ANSI/ASHRAE 52.2-1999. (ASHRAE refers to the American Society of Heating, Refrigeration, and Air Conditioning Engineers) In this system filters are evaluated based on performance in regard to several observables. Among them are Dust Spot Efficiency, Minimal Final Pressure, Particle Size Efficiency, Average Arrestance, Dust Holding Capacity and Penetration. Filters are assigned a value from 1 to 16 in increasing order of efficiency and discrimination. MERV 1 rated filters are generally throwaway materials meant for filtration of bulky mass, MERV 16 filters are capable of removing all bacteria and even fine aerosol such as smoke. In this application references may from time to time be made to filter performance as it relates to xe2x80x9cDust Holding Capacity,xe2x80x9d xe2x80x9cArrestance,xe2x80x9d xe2x80x9cPressure Dropxe2x80x9d and xe2x80x9cDust Spot Efficiency.xe2x80x9d These terms are defined as follows:
xe2x80x9cArrestancexe2x80x9d is a gravimetric measure of the ability of a tested filter to remove injected synthetic test dust from the test air, expressed as a percent of test dust by weight that a filter is able to capture. xe2x80x9cDust Holding Capacityxe2x80x9d is determined by the product of the quantity of synthetic test dust fed to the test filter, expressed in grams, and its average Arrestance. It measures the weight of test dust a filter can hold at a final pressure drop.
xe2x80x9cDust Spot Efficiencyxe2x80x9d is a measure of the ability of the filter to remove atmospheric dust from the test air. The method of determining this quantity is based upon light transmission through previously evaluated target paper. This is accomplished by adjusting the ratio of quantities of air sampled through targets upstream and downstream of the test filter so that equal changes in light transmission occur. The ratio is converted to an efficiency which is expressed as a percent. A high dust spot efficiency results in a high resistance to staining.
xe2x80x9cCakingxe2x80x9d is the build up of particulate material on the filter surface. Typically when caking occurs percent penetration decreases and Holding Capacity increases However xcex94 P also increases drastically indicating the end of the useful lifetime of the filter.
xe2x80x9cPressure Dropxe2x80x9d is a measure of a filter""s resistance to airflow and is expressed herein as xe2x80x9cDelta Pxe2x80x9d (or xe2x80x9cxcex94Pxe2x80x9d). It is a measure of the differential pressure across the filter and is usually expressed in units of inches of water. Typically a xcex94P of 1.6-1.7 inches of water is the maximum pressure drop at which point testing generally stops. It has in the past been considered a basic axiom in air filtration that as filter efficiency and filter capacity increase pressure drop also increases. It is also true that percent penetration is detrimental to filter performance and decrease in percent penetration usually results in increase in xcex94 P. Therefore, a long standing objective in air filtration research has been to increase Holding Capacity, minimize penetration, and delay the onset of caking, while keeping xcex94 P relatively constant.
Over the years a variety of techniques have been employed, and proposed to achieve the above objective. Tacking agents added to the filter have been utilized with mixed results. Among the types of materials which have been employed as tacking aids are rubber latex, polyisobutylene and a variety of viscous oils. Tacking agents tend to cause increases in pressure especially upon capturing particulates and Dust Holding Capacity drops significantly at the incidence of the caking point. This decrease in Holding Capacity in many cases is greater than the equivalent filter displays without a tacking agent. Electrostatic charging of filter substrates is often utilized. Although this type of filter exhibits some improvements in performance, it suffers from an unfortunate phenomena where it displays decrease in penetration during the early stages of loading. Additionally, oil mists and water condensation reduce the effectiveness and lifetime of the corona effect.
Now in accordance with the present invention, it has unexpectedly been found that the compositions disclosed in the present inventor""s U.S. Pat. Nos. 5,437,793; 5,698,139; 5,837,146; and 5,961,823 (all of which disclosures are hereby incorporated by reference), have extremely strong affinities for the aforementioned mist contaminants and other dispersed and/or aerosol particles in air and gas streams; and that when such streams containing these contaminant particles are passed through fluid-pervious filtration media incorporating these inventive compositions, the contaminants are immobilized at the media, as a result of which concentration levels of the contaminants in the stream filtrate may be reduced to very low values, in some instances below detectable limits in a single pass.
Filter configurations incorporating the said compositions (hereinafter referred to as xe2x80x9cabsorbent compositionsxe2x80x9d or xe2x80x9cviscolastic tackifiersxe2x80x9d (xe2x80x9cVETxe2x80x9d)), may be based on various air or gas stream permeable substrates, such as shredded, spun or otherwise configured polypropylene, polyethylene or shredded or spun cellulose, or polyester cellulose which substrates are infused or otherwise treated with the absorbent compositions, which are then cured to produce the surface modified filter. Similarly the said absorbent compositions can be incorporated into or upon other filtering substrates and media, such as paper, including compressed pulp materials, particulate porous foamed plastics, fiberglass, mineral particulates such as perlite and vermiculite, and particulate, fibrous or porous ceramic media. The resulting substrate filter may be used independently to treat an air or other gas stream from which contaminating mists or other dispersed or suspended particles are to be removed, or can be used (especially for removal of mists) in conjunction with a conventional filter, as for example by being placed in front of (i.e., in series with) the conventional filter through which the air or gas stream passes.
In general almost any conventional air or gas filtration media will be improved in performance by surface modification of the media in accordance with the invention. Filter media treated with a dry curable VET in accordance with the invention are unexpectedly found to exhibit increase Holding Capacity at little or no additional Pressure Drop and without accelerating the onset of caking. Typical tests indicate that treatment with VET absorbent compositions in accordance with the invention can improve filter performance by 2 or more MERV ratings without any significant increase in xcex94 P. Additionally it appears that the treated filter does not exhibit significant xcex94 P even after the onset of caking, further increasing Holding Capacity.
It should be appreciated that the use herein of the term xe2x80x9cabsorbent compositionxe2x80x9d or xe2x80x9cVETxe2x80x9d is one of convenience for identifying the compositions of my aforementioned patents and patent applications. The specific mechanism by which the mist, and other liquid or solid contaminant particles are removed from the gas streams by conjunctive use of the xe2x80x9cabsorbent compositionsxe2x80x9d is not completely understood, and could include attachment and/or fixation of such contaminants by mechanisms which technically involve various physical and/or chemical interactions. The terms xe2x80x9cabsorbent compositionxe2x80x9d or xe2x80x9cVETxe2x80x9d as used herein are intended to encompass all of these possible mechanisms.